Converter circuit for ac to dc with feedback means

ABSTRACT

This invention relates to a circuit for converting direct current electric power into alternating current electric power. Specifically, the invention relates to a converter utilizing a combination of simple modulation by means of feedback to produce an output containing reduced voltage variation and maintaining commutating circuit capability.

United States Patent Jackson [54] CONVERTER CIRCUIT FOR AC TO DC WITHFEEDBACK MEANS [72] Inventor: Stuart P. Jackson, 1723 Grace Lane,

Columbus, Ohio 43221 [22] Filed: July 14,1969

21 Appl. No.: 841,235

[ Feb. 22, 1972 3 ,275,948 9/ 1966 Rosenbusch .33 l/ l 81 3,416,06312/1968 Guggi ..32l/l6 FOREIGN PATENTS OR APPLICATIONS 800,122 11/1968Canada ..32 1/2 1,304,263 8/1962 France ..32 1 /45 PrimaryExaminer-William H. Beha, Jr. A ttorney-Stefan J. Klauber [52] US. Cl..32l/20, 307/103, 307/ 151 [51] Int. Cl. ..II02m 3/32 [57] ABSTRACT[58] meld of Search gig ibi E5 This invention relates to a circuit forconverting direct current electric power into alternating currentelectric power. Specifically, the invention relates to a converterutilizing a combina- [56] Reienm Cited tion of simple modulation bymeans of feedback to produce an UNITED STATES PATENTS output containingreduced voltage variation and maintaining commutating circuitcapabillty. 3,270,269 8/1966 Low ..32l/l8 X 3,388,31 l 6/1968 Delalastra..321/45 ER UX 7 Claims, 10 Drawing Figures l3 1 I4 I I7 BATTERY SWITCHOUTPUT ACTUATOR e, ACTUATOR l l I ll OUTPUT 2 3 BATTERY CONVERTER FILTERPATENTEDFEB 22 I972 SHEET 1 BF 3 TR 3 l7 BATTER f I ACTUATOT SWITCHACTL-IJ-Z'LTJOR BATTERY ERTER e2 FILTER 3 FIG 1,3 swlTcH l4 ALTERNATESENSING W BATTERY] Y SOURCE VOLTAGE RA F ACTUATOR W; L flx I6 0 D3 1 3|3o E 1- 5 D8 1r u L 7FD6 91: 1 H T 10 if I x L OSCILLATOR Fl G. 2INVENTOR.

I sTuA T P. JACKSON ATTORNEY PATENTEDFEB22 I972 3.644.819

' SHEET 2 BF 3 e2 SWITCH CLOSED SWITCH OPEN J\ ELECTRICAL Iao 360 ANGLETUATOR 3 ACFIRING SWRCE W FIG. 4

Jo PULSE FORMING -e CIRCUIT OUTPUT VOLTAGE GATE TIME To ACTUATOR DELATHYRISTOR l I O1 GATES 43 45 T FEEDBACK SIGNAL IF DESIRED FIG. 5

INVENTOR.

STUART P. JACKSON BY 3 WIN VATTORNEY PATENIEDFEB22 1912 3.644.819 sum 3hr 3 SWITCH OPEN 80 360 FIG. 6A

igmlfi a l cun h I soon 3 00' FROMG TE h H SWITCH CLOSED) 7 7 F 6 6C|aoU 360 I EEEXY HkEw-r 00* J 0 E 2 E J"' J SWITCH CLOSED I8O 360 H6 65I INVENTOR. STUART P. JACKSON ATTORNEY CONVERTER CIRCUIT FOR AC TO DCWITH FEEDBACK MEANS BACKGROUND A converter is a device which changes adirect source to one having alternating polarity components. In manyapplications it is desirable that the output waveform be sinusoidal. Forcertain types of converters this is provided naturally; such as by therotation of a coil of wire in a magnetic field as in a motoralternatorset.

There is a class of converters whose basic means of conversion isaccomplished by a switching device. The type of switch used may bemechanical (knife switches, etc.), electromechanical (contactors,synchronous switches, etc.), airgap (vacuum tube, gaseous tube, are,etc.), solid state (transistors, thyristors, etc.) or many others(light, heat, strain, liquid level, rate of change actuated, etc.)

All switching elements have in common, operation in two principle modes;i.e., open" or closed. The open condition is characterized by highimpedance between its major terminals, its closed" condition by low ornegligible impedance between its major terminals. The operatingcharacteristics of switches suggest two major applications: (I) tointerrupt current flow and (2) to provide a path for current flow.Sequential or parallel operation of sets of switches not only interruptcurrent flow to several paths at the same time and provide several pathsfor current to flow at the same time, but can also serve to reversepolarity of a direct voltage source. With the addition of multiplesources of voltage, voltage level may also be changed by interruptingsome current paths and providing for others between the various sources.

In that class of converters using switches as the major element,providing the desirable output waveform, i.e., sinusoidal, poses somedifficulties. The operating characteristics of a switch suggest thebasic output waveform of the converter is rectangular. Specifically,three basic output waveforms will appear: (I) a square wave," one whoseoutput polarity reverses twice per cycle, referred to as simple"modulation, (2) one whose output polarity reverses more than twice percycle, referred to as multiple pulse modulation, and (3) one whoseoutput may be zero for some portion of the cycle as well as of twopolarities, referred to as pulse width modulation.

In the design of a converter, it is desirable that the cost be low andwith a reliability that is high, consistent with all other applicationrequirements. This suggests that a minimum of major switching elementsbe used and that conservatively rated components and circuit redundancybe incorporated in the design. Also, it is desirable to maintain theoutput voltage of the converter within an allowable band of values. Thisis necessitated by the normal voltage drops within the converter.Voltage tolerance is aggravated by the voltage drops in the rectifiersupplying power to the converter or by the battery voltage variation (2volts/battery cell :15 percent from equalize charge to dischargecondition). In many applications, the output voltage must be held tosome reference value within :5 percent or :10 percent to simulatenormally available commercial power. It is obvious, as an example, thatif a battery supply is used, the :15 percent voltage variation from thebattery reference value must be reduced if the converter is to supply anoutput voltage within :10 percent of a reference value.

Thus there are two significant difficulties which must be solved in asatisfactory converter design for many applica tions. They are outputfiltering to produce a sinusoidal waveform and secondly to reduce thenormal voltage variation to within acceptable tolerances.

SUMMARY The present invention relates to a standby power supplyconverter circuit which operates with simple modulation of the sourcevoltage, i.e., reversing polarity of the source only. A portion of theoutput alternating wave is rectified and then added to the input of theconverter. This is done by a switching means actuated when the inputsource decreases to a preset value or when the output voltage reaches apreset value. This has the effect of increasing the necessaryvolt-ampere capacity of the converter and in maintaining the capabilityof the commutation circuit to turn the electronic switches off. The neteffect of maintaining a lower input voltage variation is of greateradvantage than the disadvantage of the converters additional handlingcapacity and in certain instances harmonic reduction in the outputwaveform occurs.

OBJECTS Accordingly it is a principal object of the present invention toprovide an improved standby power supply.

Another object of the invention is to provide a standby power supplywhich operates by means of simple modulation of its power source.

Another object of the invention is to provide a standby power supplyhaving a reduced output voltage variation.

Another object of the invention is to provide a standby power supplyhaving reduced lower order harmonics.

A further object of the invention is to provide a standby power supplywhich utilizes a minimum of major switching elements and conservativelyrated components.

Still a further object of the invention is to provide a standby powersupply which combines high reliability and low cost.

BRIEF DESCRIPTION OF THE DRAWINGS For a complete understanding of theinvention, together with other objects and advantages thereof, referencemay be made to the accompanying drawings, in which:

FIG. I is a diagrammatic illustration of the general form of thepreferred embodiment of the invention;

FIG. 2 is a diagrammatic illustration of a specific form of thepreferred embodiment;

FIG. 3 is a graphical illustration of the output voltage waveform beforethe filter, with the feedback voltage not applied (switch open) andapplied (switch closed);

FIG. 4 is a diagrammatic illustration of a typical thyristor switchwhich may be used in the embodiments illustrated in FIGS. 1 and 2;

FIG. 5 is a diagrammatic illustration of a means of periodicallyapplying the feedback voltage using the switch of FIG. 4 in theembodiments of FIGS. 1 or 2; and,

FIGS. 6a through 6e are graphical illustrations of various circuit waveshapes which may exist using the periodic application of feedbackvoltage of FIG. 5 with the thyristor switch of FIG. 4 in the embodimentsof FIGS. 1 and 2.

DETAILED DESCRIPTION OF THE DRAWINGS In accordance with the generalconcepts of the embodiments shown in the figures, simple modulationsuggests that the basic converter circuit has the capability ofreversing polarity of the source only. The other alternative remainingis to take a portion of the output alternating wave, rectify it and addit to the input source. This is done by switching means, such as anelectromechanical switch actuated when the input battery sourcedecreases to a preset value or when the output voltage reaches somepreset value.

Referring to FIG. 1, the converter 10 is supplied by a source of energy12. For the purpose of illustrating the results achievable with thepresent invention under extremely poor conditions, a battery having avariation of :l 5 percent from a reference value will be used as thesource in the preferred embodiment of a circuit of the invention. Thisrepresents the very popular application for standby power supplies usingbatteries as standby sources.

The direct voltage is applied to converter 10 through the rectifierbridge 16 comprised of diodes D1, D2, D3 and D4. In operation with theswitch 14 open, no alternating voltage, 42,, is fed to the rectifierscomprising the bridge 16. The direct voltage applied in this conditionto the converter 10 terminals 4 and 5 is the source voltage E less theforward drops of D1 and D2 in parallel with D4 and D3. It is assumed forpurposes of this description that such forward voltage drops arenegligible.

With the switch 14 closed, then the voltage e, from the converter isapplied to the bridge 16. In this case the voltage at the inputterminals 4 and 5 is E+E,, where E, is the rectified average value ofe,, assuming again that the diode drops (D1, D2, D3, D4) are negligible.

The condition of the switch 14 is determined alternatively by the sourcepotential E or the rectified value of e When the source potential Edetermines the switch 14 condition, points 1 and 2 ofTR are connected incommon. When the rectified average value of the output e determines theswitch condition, points 2 and 3 of TR are connected in common.

It should be noted that e could also serve as the sensed voltage. Inthis case, the output actuator 17 would be transferred to a point on theoutput side of the filter 19 as shown by the dotted lines.

The battery actuator 13 or the output actuator 17 may consist of a relaywhose coil is sensitive to a voltage signal causing it to change stateat a predetermined voltage. This predetermined voltage will be lowerthan the nominal value since it is the act of the actuator which causesan increase in the converter input voltage at its terminals 4 and 5.

A more complicated actuator may use a reference potential source and apolarized amplifier. When this sensed voltage reaches a preset lowervalue the reference potential could be made to be greater than thesensed potential. The difference could be used directly or afteramplification to actuate the switch 14.

A specific and practical embodiment of the invention is shown in FIG. 2.The converter illustrated in this case comprises thyristors 11 and 21,diodes, D5, D6, D7, D8, D9, D10, transformer 30 with a tapped primaryand commutating elements inductance 4 and capacitance 5 to provide turnoff potential for thyristors 11 and 21. This circuit is similar to thatdisclosed by B. D. Bedford in US. Pat. No. 3,303,406. It is to beunderstood that other converter circuits could also be used with thepresent invention. An oscillator 20 is also shown to provide timedsignals to the gates of the thyristors 11 and 21 with respect to theircathodes to cause the thyristors to conduct.

In operation with thyristor ll conducting, a steady state has beenreached, that is, current flows from the battery source 12 throughwinding 31, through both feedback winding 33 and secondary load winding31, then thyristor 11 and to source 12.

Capacitor 5 is charged to a potential equal to twice the voltage E ifswitch 14 is open (assuming diode'forward drops of D1,

sin w5t sin w3t Thus, it is seen that the fundamental is proportional tothe source potential E. Therefore, a reduction of E to aE, where a isless than 1.0 requires that the source current, I,, increase to lIi,,=(aE)(i,,/a). If, as .an example, a=0.8, then i, is increased by.l/0.8=l.25= l25 percent of its value for a=l.0. This repre- }sents anadditional current which must be commutated. However, when the currenthas increased by 25 percent due to this reduction in source potential,the energy available in the .commutating capacity to do the job ise=2C(aE) =2C(0.8E) =2C(O.64)E=l.28CE Thus a reduction in energy of 36percent has occurred.

Now close switch SW at an appropriate time to reduce the output voltagevariation by a factor of 2 (which can be done with a single switch andsingle feedback voltage). Now the source mu st supp ly this additionalcurrent which is defined by I /1: 25 percent. If a=0.8, the i, hasincreased to I25 erEHi' D2, D3, D4 to be negligible). In this condition,oscillator 20 initiates a pulse of current to the gate of thyristor 22causing it to conduct. A principal path of current flow now occursthrough capacitor 5, thyristor 22, diodes D5, D6, D7 and inductance 4.The current flow provides reverse potential across thyristor ll due tothe forward voltage drops of diodes D5, D6, D7 for a sufficient periodof time to cease conduction in thyristor 11. Another path exists forcurrent to flow through capacitor 5, thyristor 22, switch 14, diodes D1,D2, D3, D4, and transformer 30. This current is accordingly coupled tothe load winding 31 as well as the feedback winding 34. Normally theload impedances are sufficiently high to make the amount of energyconsumed from the capacitor 5 small. The capability of thecommutatingelements inductance 4 and capacitance 5 to alternatively causeconduction in thyristors 1 and 2 is dependent on the energy stored inthe capacitor 5 prior to commade common as shown. Diodes D11, D12, D13,and D14 ;then serve to rectify e,. The source 12 connected at W and Xserves as a supply for the, reference circuit and is not a part of thesensed voltage.

of its previous (switch open) value.

Thus, the current is equal to that without feedback but the minimumenergy in the commutating capacitor is i m its) which for a=0.8 equals1.62 CH Thus the energy has been FIG. 3 shows the normalized(transformer turns ratio equals unit) output voltagev The value of E, isselected to be essentially half the total natural variation of E. 3

The substitution of thyristors as a means of switching as shown in FIG.4 permits duplication of the action of the slower, switches and toinitiate the feedback potential at some electri-' Ical angle from zeroto 180. This angle a may be varied by a feedback loop to offer a smoothvariation of output voltage or i may be set at some fixed angle, aconstant, and switched on :and off as before. r The diagram FIG. 5 is ageneral presentation of a circuit to provide a fixed a gated on whenneeded by the voltage actual tor or a varying a by turning the actuatorgate on at all timesi ,and using the feedback signal on the time delaymodule. Specifically, a pulse is formed in circuit 40 by the rapid rateof r {change of e potential due to the change in polarity. The volt-;age actuator 43 by means of the gate 45, either rejects thisf 2 pulse orallows it topass to the time delay module 47. The out- I ['put pulseserves to turn on the thyristor 31 and 41 gates byi gmeans of the pulsetransformer 32 of FIG. 4. The waveforms I 5 {for the circuits of FIGS.1, 2, 4 and 5 are shown in FIGS. 6a

ithrough 62. The waveform for e in FIG. 6 shows that the volt- I iageapplied to the commutating capacitor 5 of FIG. 2 at the end of thehalf-cycle is E+E,. Thus, the energy stored is at its maximum steadystate value for the half-cycle. The harmonic content of the outputnormalized waveform :may now be analyzed by the Fourier technique sinceit is a 1 iperiodic function which is finite everywhere and has a finitei {number of maxima and minima in one period. Thus i a =angle of delay,

e(1)=fundamental= [1+ (1+cos a)] 7T 2E and 7 fl A e(3) thi rd harmonic377 [1+ 2E (1+cos 300].

TABULATED VALUES l+ cosa 1+ cosa l+cos 3a 1+ cos 311 e(-3) In: 60 3[1+0], I

thus,the additional fundamental value of 7.5 percent has been achievedwith no additional third harmonics. If the previous correction isdesired of 10 percent E,

p=0.2/l.5=0.l33 but in this case, of course, the energy stored in thecommutating capacitor is higher.

Another advantage in making oz 0, such that occurs after commutation, isthat the voltage stress on the commutating capacitors is reduced. Duringcommutation, the capacitor voltage may be in excess of twice the supplyvoltage. After commutation, this voltage reaches a steady state value ofapproximately twice the source potential. By applying E aftercommutation the peak voltage across the capacitor is more nearly equalto 2(E+E,).

In the example given it was shown that for a 0.15 feedback at a=60, thefundamental increased 7.5 percent. Note that the average input voltageincreased 6.67 percent. Thus an improvement in the ratio betweenfundamental optp ut yoltage 1 and average input voltage resulted.

it is not always necessary to make improvements by complication.Sometimes simple additions can offer advances in the art. Such is thecase with simple modulation. lncreased input voltage range capability,reduced commutating capacitor size, reduced commutating losses and,therefore, higher efiiciency and, in some cases, harmonic reduction inthe output are some of the advantages possible with this system. Reducedvoltage starting and decreased input current are also possible ifthyristors and feedback loop are em 10 ed.

Although certain and specific em 0 iments have been illustrated,it is tobe understood that modifications may be made without departing from thetrue spirit and scope of the invention.

I claim:

1. A standby power supply system the improvement comprising:

a source of direct voltage,

a converter having the output terminals of said source connectedthereto,

means to rectify a portion of the output of said converter,

and means to add said rectified portion in series to said source ofdirect voltage,

means to control the application of said rectified output to the inputof said converter, and

said control means comprising a switching means and an actuating means,

said switching means further comprising an electromechanical switch,

wherein said actuating means is connected'to and senses the power outputof said converter, said actuating means having an electrical staterelated to a predetermined value of said power output.

2. A standby power supply system as set forth in claim 1 wherein saidswitching and actuating means comprises a voltage sensitive relay.

3. A standby power supply system as set forth in claim 1 wherein saidsystem further comprises a filter circuit connected to the outputterminals of said converter and wherein said actuating means isconnected to and sensing the power output of said filter.

4. A standby power supply system as set forth in claim 1 wherein saidswitching means comprises a pulse transformer and two thyristors, saidpulse transformer having two secondary coils, said secondary coilsproviding said thyristors gates with sufficient voltage to actuate saidthyristors thereby performing the switching function.

5. A standby power supply system as set forth in claim 4 wherein saidactuating means comprises:

a pulse forming circuit having connected thereto the power output ofsaid inverter,

a voltage actuator circuit,

a gate circuit having the outputs of said pulse forming and said voltageactuator circuits connected thereto,

a time delay circuit having the output of said gate circuit connectedthereto, said time delay circuits output connected to said switchingmeans.

6. A standby power supply system as set forth in claim 5 wherein saidactuating means further comprises a feedback signal connected to saidtime delay circuit to provide a smooth variation of said systems outputvoltage.

7. A standby power supply system as set forth in claim 2 wherein saidactuating means is connected to and senses the output of said source ofdirect voltage, said actuating means having an electrical state relatedto a predetermined value of said direct voltage sources output.

1. A standby power supply system the improvement comprising: a source ofdirect voltage, a converter having the output terminals of said sourceconnected thereto, means to rectify a portion of the output of saidconverter, and means to add said rectified portion in series to saidsource of direct voltage, means to control the application of saidrectified output to the input of said converter, and said control meanscomprising a switching means and an actuating means, said switchingmeans further comprising an electromechanical switch, wherein saidactuating means is connected to and senses the power output of saidconverter, said actuating means having an electrical state related to apredetermined value of said power output.
 2. A standby power supplysystem as set forth in claim 1 wherein said switching and actuatingmeans comprises a voltage sensitive relay.
 3. A standby power supplysystem as set forth in claim 1 wherein said system further comprises afilter circuit connected to the output terminals of said converter andwherein said actuating means is connected to and sensing the poweroutput of said filter.
 4. A standby power supply system as set forth inclaim 1 wherein said switching means comprises a pulse transformer andtwo thyristors, said pulse transformer having two secondary coils, saidsecondary coils providing said thyristors'' gates with sufficientvoltage to actuate said thyristors thereby performing the switchingfunction.
 5. A standby power supply system as set forth in claim 4wherein said actuating means comprises: a pulse forming circuit havingconnected thereto the power output of said inverter, a voltage actuatorcircuit, a gate circuit having the outputs of said pulse forming andsaid voltage actuator circuits connected thereto, a time delay circuithaving the output of said gate circuit connected thereto, said timedelay circuit''s output connected to said switching means.
 6. A standbypower supply system as set forth in claim 5 wherein said actuating meansfurther comprises a feedback signal connected to said time delay circuitto provide a smooth variation of said system''s output voltage.
 7. Astandby power supply system as set forth in claim 2 wherein saidactuating means is connected to and senses the output of said source ofdirect voltage, said actuating means having an electrical state relatedto a predetermined value of said direct voltage source''s output.